Spinal implants constitute 40% of all orthopedic implants, with pedicle screws among the most frequently implanted devices. Spinal fixation systems are used in surgery to align, adjust, and fix the spinal column, many times by fusing vertebrae together. Typical spinal fixation systems use a rod or plate to support the spine and to properly position the vertebrae. Anchors, consisting of pins, bolts, screws, and hooks are used to engage the vertebrae and support the rod or plate and are driven into the pedicles of the spine. While the rod or plate varies in size, length, and shape, the rod is generally modified before insertion into the anchor, due to the complex nature of spinal anatomy. In many instances, two or more non-aligned anchors require bending of the rod or plate, thereby introducing fatigue resistance into the rod or plate.
Currently, there are several models of screws on the market, of which there are polyaxial screws and monoaxial screws. Polyaxial screws can be locked/rigid (monoaxial) or variable angle (polyaxial) based on simple intraoperative manipulations to change the stability of the top loading (tulip) end of the screw. A new category of polyaxial screws has been developed by several companies that allow the screw to change from variable angle to rigid at the will of the surgeon. These screws, however, lack the ability to be manipulated surgically in a sagittal direction, limiting a surgeon's ability to manipulate the vertebral bodies to create sagittal alignment.
The rod attaches to the screw at the body, either using a top loading or side loading arrangement. Most conventional top-loading polyaxial screws do not address cantilever failure and fail to permit adequate flexibility because the rod or plate rests too close to the screw rotational center. Placing a rod into side-loading screw requires significant forces to insert the rod into the screw body if the screws are not perfectly aligned. As such, many side-loading systems require two surgeons to reduce the spinal rods into the screw settings prior to tightening. This can also lead to fracture of the vertebral body/pedicle by displacement of the screw, weakening of the construct or lateral rotation of the vertebral body leading to a rotational deformity which is not intended.
The TSRH SiLo 5.5 Spinal System Sagittal Adjusting Screw (SAS) was designed to combine the correction of a fixed angle screw with a multi-axial screw, reducing the stress incurred on the bone during correction maneuvers. The SiLo 5.5 Spinal System also features a device that allows surgeons to use one hand in securing rods to pedicle screws and provides greater reduction than current systems. However, once the SiLo screw is rigid, the SiLo screw also requires exact alignment for placement of the rod. It is not possible to manipulate the screw relative to the rod in a sagittal direction for manipulation of the vertebral bodies, whereas in a trauma situation, it is frequently desirable to manipulate the vertebral bodies to correct a kyphotic deformity due to a burst fracture.
The Depuy EXPEDIUM screw has an inner and outer head screws which allows sequential tightening of either the polyaxial rotational component of the head or tightening of the screw onto the rod. This allows the surgeon to either distract independently or make the screw rigid versus polyaxial on an independent basis. However, the EXPEDIUM screw cannot be manipulated in a sagittal direction in relation to the rod, which is frequently required to change sagittal malalignment or imbalance. Examples of this include trauma situations with burst fractures managed from posterior surgical approaches, as well as flat back cases where increasing sagittal lordosis may be desired, among others.